The present invention relates to grinding machines and more particularly to an apparatus for controlling grinding machine parameters as a function of grinding force exerted upon the grinding wheel.
It has become common practice in present day grinding machines to employ multimodal grinding cycles to reduce cycle time and enhance machine productivity. The typical cycle includes at least three distinct modes, each with different rates of grinding wheel advancement. The first mode in a typical grinding cycle sequence is the rapid advance mode, in which the grinding wheel is moved toward the workpiece from a machine loading position to a selected location therebetween. The purpose of the rapid advance mode, as the terminology suggests, is to rapidly move the wheel toward the workpiece in order to expedite the grinding cycle time. In an external grinder, this movement is generally in a direction transverse to the axis of the workpiece and powered hydraulically. In an internal grinder, the rapid advance generally comprises two movements; first, the table slide motion parallel to the workpiece axis which moves the grinding wheel into the workpiece cavity; and second, the cross slide movement which is transverse to the workpiece axis. The second, or gap eliminator mode commences with the termination of the rapid advance mode at the selected location. It is in this gap eliminator mode in which initial workpiece engagement is reasonably anticipated. The grinding wheel velocity in the second mode, frequently powered by an electrical stepping motor, is generally slower than the rapid advance rate but still in excess of that suitable for the grinding process. Since the wheel advance velocity during the second mode is excessive, provisions must be made for reducing the velocity after workpiece engagement in order to perform the actual grinding.
Prior art machines have performed this intermodal transition in several different ways. One method is to rapidly move the grinding wheel (or the workpiece if it is being moved to effectuate the relative movement with the grinding wheel) to a predetermined fixed position close to the inital engagement point. The third, or grinding mode is then initiated and the velocity is reduced to a lessor level which is suitable for grinding. A second more sophisticated method is that of force detection. In this method, a gap eliminator circuit monitors the grinding force by measuring electrical load upon the grinding wheel motor. A sudden increase in the current is then taken as an indication that workpiece engagement has occurred, and a signal is generated in response thereto and used to reduce the wheel velocity. It is in accordance to this latter method in which the apparatus of the present invention controls the grinding machine.
In the force detection method, it is desirable to reduce the wheel velocity as soon as possible after contact with the workpiece, which requires that the change in velocity occur with a small increase in motor current. In prior art machines, the grinding wheel current, which is approximately proportional to force, is biased by a fixed amount representing idle current in the motor. However, as a practical matter the idle current is not constant but varies with many factors including, inter alia, grinding wheel size, the drive train between the motor and the wheel, power surges, parts wear and temperature changes. Consequently, the idle current bias is set to the worst case situation which is too high for the best gap eliminator operation in most cases. Further, reducing the bias results in undesirable accidental triggering of the gap eliminator circuit.
It also sometimes happens that the grinding wheel makes contact with an object during a very early part of the grinding cycle, as for example, in the rapid advance mode. This generally represents a gross misoperation of the grinding machine and continued advancement of the wheel will frequently operate to the extreme detriment of the machine. Experience has shown that the best course of action in such circumstances is to retract the grinding wheel. Prior art grinders have attempted to solve this problem with an "interference circuit". The "interference circuit" monitors grinding force and is analogous to the gap eliminator circuit described above. It retracts the wheel whenever "interference" is encountered in the rapid advance mode. These attempts have been plagued by the same problems confronting the gap eliminator circuit and have generally worked independently of the gap eliminator.
The present invention presents a novel and unique solution to the aforementioned difficulties. A single cycle modification signal is utilized by both the interference and gap eliminator circuits, and the idle current bias is modulated on an updated basis with each new grinding cycle. Many of the time-dependent variables in the idle current compensation signal are eliminated, and the control's efficaciousness is improved markedly.